This application is based on application No. 10-1999-56718 filed in the Korean Industrial Property Office on Dec. 10, 1999, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a negative active material for a lithium secondary battery and a manufacturing method of the same, more particularly to a negative active material for a lithium secondary battery having superior high rate cycle life characteristics and a manufacturing method of the same.
(b) Description of the Related Art
Carbonaceous materials which are used as a negative material of a lithium secondary battery can mainly be classified into amorphous carbon and crystalline graphite, depending on the crystallinity of the substance. Crystalline graphite, which is more generally used out of the two materials, can again be classified into artificial graphite and natural graphite. Typical examples of artificial graphite include mesocarbon microbeads (MCMB) and mesocarbon fiber (MCF), and these are typically used in lithium secondary batteries.
Although natural graphite has a quite high crystallinity thus having a very high initial discharge capacity, it produces a flake shaped material during the crushing process because of its high crystallinity, and this causes an irreversible capacity increase at the edge of the natural graphite. In addition, natural graphite is severely compressed during electrode preparation, making it very difficult to secure an electrolyte penetration path between particles of active material, and a lithium ion transfer pathway is elongated compared with artificial graphite based active materials. Therefore, cycle life characteristics of natural graphite, particularly high rate cycle life characteristics, are inferior to artificial graphite of a spherical, fibrous, or amorphous form.
A method for improving cycle life of a battery by increasing conductivity between active materials is disclosed in Japanese Patent Laid-open Publication Nos. Hei 9-161776, Hei 8-7895, and Hei 8-69797. This is a method in which metallic particles such as Cu etc. are non-electrolytically plated on a graphite surface layer, or particulates such as Ni, Fe, etc. are mixed with an active material to be used as a negative pole plate. However, the method for plating metallic particulates on a graphite surface has little effect upon the form of the active material, thus it is limited in improving conductivity on a pole plate having high density for providing high capacity, and uniform mixing is practically impossible due to density differences among each material during the preparation of a large amount of slurry in the method for mixing graphite, carbon powder, and metallic particulates.
It is an object of the present invention to provide a negative active material for a lithium secondary battery which has superior high rate cycle life characteristics even with a high pole plate density by improving conductivity between active materials.
It is other object of the present invention to provide a method for manufacturing the negative active material for a lithium secondary battery.
In order to accomplish the above described objects, the present invention provides a negative active material for a lithium secondary battery including crystalline or amorphous carbon on which surface catalysts are doped or dispersed, and carbon vapor growing fiber or carbon nanotubes are grown.
Furthermore, the present invention provides a method for manufacturing a negative active material for a lithium secondary battery. In this method, crystalline or amorphous carbon is mixed with a catalyst element or compound thereof to produce a crystalline or amorphous carbon on which a surface catalyst element or compound thereof is doped or dispersed. The crystalline or amorphous carbon on which the catalyst element or compound thereof is doped or dispersed; are then carbonized. Thereafter, the carbonized crystalline or amorphous carbon is vapor deposited at a temperature of 300 to 1500xc2x0 C. under the existence of carbon-containing gas to form carbon vapor growing fiber or carbon nanotubes on a surface of the crystalline or amorphous carbon.